1. Field
The disclosure relates generally to a voltage reference circuit and low voltage oscillator and, more particularly, to a system for a low power consumption thereof.
2. Description of the Related Art
Voltage reference circuits and oscillator circuits consume power which impacts the total system power consumption. Voltage reference circuits and oscillators are used in conjunction with semiconductor devices, integrated circuits (IC), and other applications. The requirement for a stable reference voltage is often required in electronic design. Voltage reference circuits that provide a stable reference voltage are sometimes bandgap voltage reference circuits.
A traditional bandgap reference circuit, the voltage difference between two p-n junctions (e.g. diodes, or bipolar transistors), operated at different current densities or at different transistor sizes, can be used to generate a proportional to absolute temperature (PTAT) current in a first resistor. This current can then be used to generate a voltage in a second resistor. This voltage, in turn, is added to the voltage of one of the junctions. The voltage across a diode operated at a constant current, or herewith a PTAT current, is complementary to absolute temperature (CTAT). If the ratio between the first and second resistor is chosen properly, the first order effects of the temperature dependency of the diode and the PTAT current will cancel out. In this fashion, a circuit can be independent of temperature variation, and provide a constant voltage reference.
Circuits of this nature that are temperature insensitive are referred to as bandgap voltage reference circuits. The resulting voltage is about 1.2-1.3V, depending on the particular technology and circuit design, and is close to the theoretical silicon bandgap voltage of 1.22 eV at 0 degrees Kelvin. The remaining voltage change over the operating temperature of typical integrated circuits is on the order of a few millivolts. Because the output voltage is by definition fixed around 1.25V for typical bandgap reference circuits, the minimum operating voltage is about 1.4V. A circuit implementation that has this characteristic is called a Brokaw bandgap reference circuit.
In voltage reference circuits, operation below the bandgap voltage level is desirable. These voltage reference circuits are known as sub-bandgap voltage references. Technology scaling of the physical dimensions of integrated electronics allows for higher density circuits. To maintain reliability of semiconductor components, dimensional technology scaling also requires scaling of the power supply voltage. This is known as constant electric field scaling theory. But, with the technology scaling, silicon remains the most commonly used technology. Hence, voltage reference circuits with power supply voltages as low 1.1 V will require sub-bandgap operation.
With mixed voltage interfaces, it is also desirable to provide voltage reference circuits and oscillators above the bandgap voltage level. Voltage reference circuits and oscillators that operate to 3.6V power supply levels are also needed. Systems power supply rail voltages can range from 1.1V to 3.6V. In semiconductor technologies, typically having at least two transistors, with different MOSFET gate oxide thickness, typically referred to as thin-oxide MOSFET and thick oxide MOSFET. The thick oxide MOSFET uses dual oxide, or triple oxide thicknesses to provide higher power supply voltage tolerance for higher voltage operation and applications. Voltage tolerance for circuits can also be achieved by using “stacks” of MOSFETs (for example cascode MOSFET circuits) to lower the voltage across any given thin-oxide MOSFET transistor.
In some system applications, voltage reference and oscillators can be turned on, and turned off, sequence dependent, sequence independent, as well as “always on” systems. In the case of an “always on” system, power consumption is an issue. It is desirable to have voltage reference and oscillators in an “always on” state which has a low power consumption. A target level for low power consumption is typically 3 μA for the portable business.
A prior art sub-bandgap voltage reference is depicted in FIG. 1. A low voltage power supply rail voltage VCC 10 and a ground rail 20 provides power to the circuit. The voltage reference output 30 is between the power supply voltage and the ground potential. The differential amplifier 40 supplies an output voltage to the gates of p-channel MOSFET P1 50A, P2 50B, and P3 50C. The p-channel MOSFET P1 50A drain is connected to the parallel combination of resistor R1 60, and diode 70. The p-channel MOSFET P2 50B is connected to an array of diode elements 80 and a resistor R3 90. A second resistor R2 95 is in parallel with the array of diodes 80 and resistor R3 90. The output reference voltage 30 is electrically coupled to the p-channel transistor P3 50C and resistor element R4 97.
In the prior art circuit of FIG. 1, the technology that is used only allows for a “stack” of one MOSFET gate-to-source voltage, VGS, and one MOSFET drain-to-source voltage, VDS for a low voltage rail. Furthermore, the power supply rail is as low as 1.1V, traditional bandgap voltage reference networks can not be used. Different implementations can not only use this network, but also operational amplifiers. Operational amplifier circuit topologies always need a at least one MOSFET drain-to-source voltage, VDS, for tail current generation, one MOSFET drain-to-source voltage, VDS, for the V-mode comparison, and one MOSFET gate-to-source voltage for the output p-channel MOSFET (PMOS) to drive. As a result, these structures are not suitable for a minimum power supply voltage of 1.1V. Operational amplifier circuits add significant increase in the number of circuit branches, leading to more complexity, more complications, and more power consumption.
For oscillator circuits, low power consumption and accuracy are important design objectives. FIG. 2 illustrates a relaxation oscillator circuit. The circuit is powered by VCC 150. A comparator 100 evaluates two incoming signals from the voltage on a capacitor VC 105 with respect to a reference voltage VREF 110. The current reference IREF 120 provides current for the charging of the capacitor C 140. A switch 130 is activated by a feedback loop from the COMPOUT (oscillator out signal) 160. The switch 130 is in parallel with capacitor 140. The comparator adds at least four branches to the circuit (e.g. a differential pair, a bias and an output stage). Additionally, it requires a voltage of a MOSFET gate-to-source voltage VGS, and two MOSFET drain-to-source voltages, 2 VDS, to operate properly. Hence, this limits the ability to use this circuit for sub-bandgap voltages, and low voltage applications. The generation of IREF 120 requires an extra operational amplifier to divide the voltage reference by a resistance of value R. Hence, the oscillator introduces a number of current branches increasing the complexity of the network.
With technology scaling, according to constant electric field scaling theory, the power supply voltage, VDD, continues to decrease to maintain dielectric reliability. In current and future semiconductor process technology, having minimum dimensions of, for example, 0.18 μm, and 0.13 μm, the native power supply voltage (or internal power supply voltage) is 1.5V internal supply voltage for digital circuits, and other sensitive analog circuitry. For technologies whose minimum dimension is below 0.13 μm, the issue is also a concern.
In oscillators, a low voltage wide frequency oscillator has been described. As discussed in U.S. Patent Application US 2013/0229238 to Wadhwa describes a low voltage oscillator that is controlled by latch networks. The implementation includes multiple delay elements, in which each delay element includes two inverters, a control input, a plurality of delay elements, a latching element, and a plurality of current-source devices.
Low power oscillators have been disclosed. As discussed in U.S. Pat. No. 8,390,362 to Motz et al, a low power, high voltage integrated circuit allows for both low power, and high voltage in a given implementation. The circuit controls a sleep/wake mode, or a duty cycle.
Low voltage oscillators can utilize capacitor-ratio selectable duty cycle. As discussed in U.S. Pat. No. 7,705,685 to Ng et al., discloses an oscillator operating at very low voltage yet has a duty cycle set by a ratio of capacitors, with an S-R flip-flop latch that drives the oscillator inputs.
Low voltage bandgap voltage references utilize low voltage operation. As discussed in U.S. Pat. No. 5,982,201 to Brokaw et al., a low voltage current mirror based implementation shows a bipolar current mirror network, a resistor divider network, an output transistor that allows for operation with supply voltages of less than two junction voltage drops.
A low voltage oscillator can also have oscillation frequency selection. As discussed in U.S. Pat. No. 4,591,807 to Davis, describes a low power, fast startup oscillator circuit comprising of an amplifier, a current mirror, a feedback biasing means, and a tuned circuit for selecting the frequency of oscillation.
In these prior art embodiments, the solution to improve the operability of a low voltage bandgap circuit and oscillators utilized various alternative solutions.
It is desirable to provide a solution to address the disadvantages of the low voltage operation of a bandgap reference circuits and oscillators.